Cause Several fungal leaf spots occur on alfalfa. The most prevalent ones in the Pacific Northwest are common leaf spot caused by Pseudopeziza medicaginis, rust caused by Uromyces striatus, Stemphylium leaf spot caused by several species of Stemphylium (teleomorph = Pleospora spp.), and Stagonospora leaf spot and root rot caused by Stagonospora meliloti (=Phoma meliloti; teleomorph = Leptosphaeria pratensis).
Symptoms
Pseudopeziza leaf spot-small, more or less discrete circular spots with a toothed edge.
Rust-small reddish-brown circular pustules (uredinia) on both leaf surfaces. Elongate pustules may occur on stems. Plants may defoliate prematurely. Alternate host is leafy spurge or cypress spurge.
Stagonospora leaf spot-leaf spots are circular or angular and pale buff with a light-brown margin.
Stemphylium leaf spot-leaf spots are irregularly shaped, brown, with a darker border surrounded by a yellow halo. It commonly develops only in wet, cool weather.
Cultural control
- Plant adapted recommended varieties.
- For some leaf spots, resistant varieties are available.
- Cut early when the disease is severe.
Chemical control Control with fungicidal sprays is possible for some of these diseases and may benefit seed crops.
- Copper (Group M1) materials can aid but beware of buildup of copper in soils.
- Champ Formula 2 at 1.33 pints/A, 10 to 14-days prior to harvest. 48-hr reentry.
- Kocide 2000 at 1.5 lb/A, 10 to 14-days prior to harvest. 48-hr reentry. O
- Nordox 75 WG at 1.25 to 2.5 lb/A, 10 to 14-days prior to harvest. 12-hr reentry.
- Previsto at 0.75 to 2 quarts/A. Apply 10 to 14 days before each harvest or earlier; 30-day retreatment interval. 48-hr reentry. O
- Priaxor Xemium (Group 7 + 11) at 4 to 6.9 fl oz/A on 14- to 21-day intervals. Do not make more than 2 applications per cutting or 3 applications per year. Preharvest interval is 14 days. 12-hr reentry.
- Quadris Flowable (Group 11) at 6 to 15.5 fl oz/A. Do not apply more than 15.5 fl oz/A per cutting. Preharvest interval is 14 days for grazing or harvest for forage and hay. 4-hr reentry.